The overall goal of this proposal is to elucidate the mechanisms of animal development rough the study of the nematode Caenorhabditis elegans, a model organism chosen for its relative cellular simplicity and its advantages for classical and molecular genetic analysis. An understanding of the genetic basis of development may well be fundamental to much of medicine and may ultimately contribute important information to many problems, ranging from congenital defects to senescence. Five specific aims are proposed. The long-term objective of the first four specific aims is to identify and characterize the molecular mechanisms of action of a large network of genes that interact, directly or indirectly, with the C. elegans mec-8 gene, which encodes a protein with two RNA recognition motifs and regulates the processing of transcripts of several genes, including unc-52. Besides using genetic approaches to expand a gene network, methods for studying genetic redundancy in C. elegans are introduced. The first specific aim is to identify and characterize mutations that are synthetic lethal with mec-8 loss-of-function mutations. Such mutations, called sym for synthetic lethal with mec-8, define genes that functionally overlap mec-8. Mutations in sym genes by themselves confer essentially no mutant phenotype but in combination with a mec-8 mutation are lethal. The second aim is to identify by mutation additional genes that interact with sym-1, which encodes a protein that is secreted from the apical surface of the embryonic hypodermis and plays a role in the attachment of body muscle to cuticle. Mutations that are synthetic lethal with a sym-1 mutation will be sought, as will suppressors of the synthetic lethality of mec-8 and sym-1 mutations. The third specific aim is to identify and characterize molecularly several of the genes that interact with mec-8. Four such genes have already been defined genetically; others will be identified under the first two specific aims. The fourth specific aim is to characterize the effects of mec-8 and two other genes, defined by mutations that suppress mec-8 mutations, on the processing of unc-52 pre-mRNA. The long-term objective of the fifth specific aim is to learn more about the assembly and maintenance of eukaryotic cilia and flagella. Mutations to ten C. elegans genes that affect the ability of living animals to incorporate a fluorescent dye into a set of sensory neurons will be characterized further in order to identify those genes that affect assembly of sensory cilia. Null alleles of such genes will be sought.